The present invention relates to an IC socket used for establishing reliable connection between electrode terminals of an IC, and a wiring board or the like connected to inspection equipment at the time of inspection of a monolithic IC or a hybrid IC, such as an LSI (a large-scale integrated circuit), or a module component into which a plurality of discrete components, such as ICs and LCRs, are integrated to perform a required function (the ICs and the module components are hereinafter collectively referred to simply as an “IC”).
More specifically, the present invention relates to an IC socket capable of connecting a high-frequency, high-speed IC (an analog high frequency is called a high frequency; a digital frequency having a very short pulse width and a very short pulse interval is called high-speed and both frequencies are hereinafter collectively referred to as radio frequency “RF”) in which terminals are arranged with a very narrow pitch of about 0.4 mm, so that the IC can transmit a signal without fail.
Characteristics of a recent IC which has been highly integrated and has attained high performance must be inspected before it is actually mounted on a circuit. When such an IC is inspected, wiring terminals of a wiring board which is connected to inspection equipment and has wiring patterns formed thereon must be reliably connected to electrode terminals of an IC without use of soldering. Therefore, an IC socket employing leaf springs or contact probes provided on an insulative base is used. Particularly, in the case of a recent IC, in which the number of electrode terminals is increased and intervals become smaller, the IC can be preferably connected by way of slender leads through use of contact probes which require smaller contact areas.
FIG. 9 shows an IC socket 70 using such contact probes. In the IC socket 70, contact probes 72 are arranged in a matrix manner so as to correspond to electrode terminals 73a of an IC 73 mounted on a plastic or ceramic base 71. Each of the contact probes 72 is configured such that a pair of plungers are held in a metal pipe by way of a spring, so that the plungers are retractably projected from both ends of the metal pipe.
When the IC 73 is pressed against one side of the IC socket 70 while being positioned by a guide hole 71a, reliable contacts are established between the terminals 73a and one ends of the contact probes with the aid of the elastic retraction of the plungers. The other side of the IC socket 70 is mounted on a wiring board 75 on which wirings connected to the inspection equipment are provided, and fixed thereon through use of screws or the like. The contacts between the other ends of the contact probes and the wirings can be also reliably established with the aid of the elastic retraction of the plungers.
However, the recent IC is susceptible to a higher-frequency and higher-speed signal as well as to high integration and high performance. In the case of a narrow lead such as a contact probe, transmission of an RF signal of 1 GHz or higher is hindered by a reactance component of the lead, or influence of signal reflection cannot be ignored. For instance, even when contact probes are shortened to a size of the order of about 2 mm in order to reduce an inductance component, difficulty is encountered in reducing the reactance component of the contact probes to 1 nH or less. For instance, a probe of 1 nH yields impedance of 63 Ω at 10 GHz.
In order to solve such a problem, in a jig for inspecting a module device, there is conceived a coaxial structure, in which a metal block serves as a base and contact probes are inserted into through holes by way of dielectric tubes (see Japanese Patent Publication No. 2001-99889A).
As mentioned previously, in association with the frequency and speed of an IC having been increased, a conventional IC socket formed by setting contact probes upright on an insulative base suffers inhibition of signal transmission such as distortion of a waveform due to attenuation or reflection, to thus fails to perform accurate inspection. For this reason, realization of a probe of coaxial structure is also conceivable.
However, in order to achieve predetermined impedance through use of the coaxial structure, an outer diameter “d” of each of a plurality of contact probes serving as core conductors and an inner diameter “D” of a hole of a metal block serving as an outer conductor must satisfy the relationship expressed by Equation (1) while a relative dielectric constant of a dielectric substance interposed between the contact probes and the metal block serves as εr.                     Zo        =                              60                                          ɛ                r                                              ⁢                      log            e                    ⁢                      D            d                                              (        1        )            
Therefore, even when an attempt is made to realize a coaxial structure of 50 Ω through use of a tube formed from polytetrafluoroethylene, which is known as a dielectric having a small relative dielectric constant on the order of 2.1, the inner diameter D of a hole formed in the metal block must be about 3.3 times as large as the outer diameter “d” of the contact probe.
A high-frequency, high-speed device consumes a larger amount of electric current in association with a recent increase in the degree of integration of an IC, and a very large number of earth terminals and power terminals are formed with a view toward decreasing the DC resistance and high-frequency impedance of the earth terminals and those of the power terminals. In some cases, about 600 electrode terminals/cm2 are provided, and there has been developed an IC having a narrow pitch between electrode terminals, such as a pitch of about 0.4 mm. In order to impart the coaxial structure to contact probes which come into contact with electrode terminals provided at a narrow pitch of about 0.4 mm, the contact probes may fail to be compatible with an IC having a narrow pitch of about 0.4 mm unless the outer diameter of the contact probe is reduced to 0.1 mm or less.
However, the contact probes have a structure in which a spring and a plunger are inserted into a metal pipe. If the outer diameter of each of the contact probes is reduced excessively, the contact probes become very costly. Further, there is raised a problem of a drop in durability and reliability.